Delayed coking operations involve thermal cracking of heavy liquid hydrocarbons to produce various more valuable hydrocarbon fractions and coke. Any suitable delayed coking feedstock can be used as starting material, including liquid vacuum resid from a crude oil refining process.
Delayed coking is generally carried out by initially heating a liquid feedstock in a coking furnace to a coking temperature, often between 875.degree. F. and 950.degree. F. The furnace includes a coil of multiple furnace tubes wherein the feedstock is heated before passing to a coker drum. The furnace tubes of an operating furnace include temperature and pressure gradients along the coil. Thermal cracking of the heated feedstock occurs primarily in the coking drum to yield mixed volatile hydrocarbon vapors and coke. The vapors are drawn off overhead and introduced to a fractionator column wherein hydrocarbon fractions including gases, gasoline, one or more distillate streams and a heavy gas oil stream are separated and subsequently isolated.
There are several operating requirements and objectives that are normally considered in developing and optimizing a delayed coking process. It has often been sought to efficiently minimize the overall coke yield while also minimizing fouling of the radiant coker furnace tubes by premature coking. Fouling of the internal walls of the furnace tubes can cause blockages requiring periodic operation shut-downs to clear the lines. Modern delayed coking operations present the potential for rapid tube fouling due to increased feed rates and increased concentrations of fouling components in feedstocks such as asphaltenes, inorganics and heavy metals.
It has been found that a useful coker furnace operation preferably includes blending with the feedstock a natural heavy recycle material fed from the fractionator bottom. The recycle improves the operation of the furnace and provides a solvent effect to the feed that aids in reducing fouling (coking) on the walls of the furnace tubes. Depending on the particular coking system, the recycle material has a boiling range from as low as 400.degree. F. to over 1000.degree. F. However, a competing consideration is that the recycle material can potentially increase the overall coke yield in the coke drum at the expense of other more desirable hydrocarbon products.
The fractionator column for flash distillation in a delayed coking unit is typically equipped with a spray down source of heavy gas oil for condensing hydrocarbon vapors entering the fractionator from the coker drum. Heavy gas oil can also be used to quench vapors leaving the coke drum. The spray down and quench can be fed directly from a heavy gas oil collection tray on the column and comprise at least part of the natural heavy recycle to be combined with the feedstock.
Improvements in coker fractionators have led to reductions in the quantity of heavy spray down material required for proper fractionator operation. Modern feedstock compositions also frequently inherently require less quench and spray-down because less gas product is produced. In response, a supplemental hydrocarbon diluent fed to the furnace in addition to the natural heavy recycle has been proposed, such as in the case of coking a resid from a low gravity crude. The diluent can be provided as forced recycle diverted directly from a fractionator product stream. Due to the increased potential for coke formation, however, prior art methods have sought to avoid higher boiling diluents or additives, such as heavy gas oil, and have sought to reduce the recycle rate.
With an objective of reducing overall coke yield, U.S. Pat. No. 4,455,219 discloses a method of minimizing heavy recycle in a delayed coking operation and blending the coker furnace feed with a forced recycle of a lower boiling distillate stream taken from an intermediate fractionator tray, most preferably with a boiling range of from about 510.degree. F. to 650.degree. F.
Similarly, in providing a diluent material to aid furnace operation, U.S. Pat. No. 4,518,487 discloses a process for reducing coke yields by eliminating the natural heavy recycle and substituting an intermediate distillate as a recycle component. It is said that elimination of the heavier components of the furnace feed is especially important. Therefore, a forced recycle component having a boiling range upper end below 850.degree. F. is said to be required.
U.S. Pat. No. 4,549,934 discloses a process for removing condensed coke drum vapors from a fractionator to completely prevent their appearance as recycle in the furnace feed. A recycle material lighter than heavy gas oil is substituted into the furnace feed.
U.S. Pat. No. 5,645,712 discloses a process having a purpose of reducing coke yield wherein light gas oil from a coker fractionator is separately heated and directly introduced to a coking drum.
Prior art processes have been primarily concerned with reducing overall coke yields by minimizing heavy recycle rates and substituting distillates such as light gas oil for all or a portion of the natural heavy recycle. Unfortunately, the prior art has not recognized that light distillates do not provide optimal results in a delayed coking operation, especially in terms of furnace operation efficiency. Modern delayed coking operations now often work at higher feed rates with feedstocks containing higher concentrations of carbon residue and other fouling components. Therefore, the prior art methods of supplementing a coker feed with a distillate stream are often insufficient.
It would therefore be desirable to have an improved delayed coking apparatus and process without the shortcomings of the prior art. The present invention provides a delayed coking process and apparatus including superior reduction in furnace tube fouling rates without appreciable increases in coke yields as compared to prior art processes. This involves supplementing the natural heavy recycle with full boiling range heavy gas oil, such as that taken directly from a coker fractionator heavy gas oil stream. The invention further provides a method and apparatus for injecting a coker feed additive in order to maximize its anti-fouling effectiveness. Other aspects of the invention will be apparent from the following summary and detailed description of the invention.